Extruded metal exterior window and door casing, generally made of aluminum are widely used in construction of all kinds and particularly in office and industrial buildings. Such metal casings are extremely good thermal conductors and therefore can cause considerable heat loss in winter and heat gain in summer in a building in which they are installed. To reduce this problem it is becoming conventional to employ a "thermal break" between the interior and the exterior portions of a metal casing. The thermal break is made of a material of relatively low thermal conductivity and serves to interrupt the transfer of heat between the interior and exterior metal portions.
Conventional process from manufacturing a thermal break is to provide a metallic casing extrusion with a central top opening channel into which a liquid curable polymer formulation is poured and cured. After curing the bottom of the channel is milled off so as to break the continuity of the extrusion, leaving a cured solid polymer intermediate segment bonded between two separate metal segments. It also conventional to use curable polyurethane catalyzed with a metal catalysts such as dibutyltin dilaurate as the liquid curable polymer material.
In use such polyurethane thermal breaks are subjected to high stresses caused by day, night and seasonal thermal cycling of the metal segments which have much higher thermal expansion coefficients than the polyurethane thermal break. Further, these stresses are different on each side of the break due to differential interior and exterior temperatures. Consequently, it is not uncommon for the break to debond from the metal segments of the casing resulting in a loss of structural integrity.
There is therefore a need for a metal casing structure having a polyurethane thermal break which displays improved structural integrity upon thermal cycling.